As counterintuitive as it is, マ ニアック would be more polite, or as noted above, fan. If you're just getting to know someone and asking about their interests, it would be best to say something like "oh, so you're an anime fan?" rather than otaku or even maniac. If you know someone reeeallly is an otaku, but still want to be nice, you could say "maniac" instead, and also it would probably seem more normal if you referred to yourself as a "maniac" rather than "otaku" if that's the case.